1. Field of the Invention
The present invention relates generally to animation and more specifically to computer animation of characters. The present invention is also directed to providing animators with greater control over aspects of the character to produce reasonable changes in those aspects even when the character undergoes unrealistic motion.
2. Description of the Related Art
Traditional animation techniques allow the animator to create the apparent motion of animated characters, to be viewed by a viewer of the animation. The use of computers to simplify the animation process has provided many benefits to the traditional hand-drawn process. Computer animated characters are well known in the prior art and have been used in many different capacities. Such characters are utilized in traditional movies, videos and online streaming of moving pictures, as well as interactive movies where the motion of characters is often initiated by a user.
Often times in the animation of characters, the characters have “secondary elements”, such as clothing and hair, that are responsive to main motion of the characters. The motions of some secondary elements in computer graphics imagery are often too complex for an animator to directly control. Instead of a human animator determining the motion of these secondary elements, computer programs use physically-based numerical methods that simulate the motion of these secondary elements (such as hair or cloth) over time.
This is accomplished by modeling the physical properties of these dynamic elements (how the cloth bends due to forces or collisions with solid objects, how the hair deforms or collides with itself), the external forces on these elements (gravity, wind) and the motions of the kinematic elements (the characters that cloth rests on or that the hair grows out of). The animation of the kinematic elements is provided by the animator and is independent of and unaffected by anything that the dynamic elements do.
For concreteness, consider an animation of a superhero wearing a long cape: The superhero (the kinematic element, Captain IFG), FIG. 1a, is directly animated by a skilled human animator, while the motion of the superhero's cape (the dynamic element) in response to the superhero's animation is simulated using physically-based numerical techniques.
If the physical properties and external forces acting on the dynamic elements are accurately modeled, the resulting motion will be plausible and seemingly realistic: cloth will hang down and fold naturally, hair will droop or blow in the wind. However, it is often the case that the kinematic elements (the primary characters of the scene) may be animated in a physically exaggerated manner, yet this requires the dynamic elements to essentially ignore this exaggeration and continue to behave in a realistic manner.
As an example assume the superhero stands motionless for a moment, then abruptly accelerates upward at 100G's (98,000 cm/s2). See FIG. 1b. His cape, tied around his neck, drapes down his back while the superhero is motionless, but must then react to the superhero's enormous upward motion. The result of the simulation program in this situation is to accelerate the portions of the cape around the superhero's neck along with the superhero.
The rest of the cape, unfortunately, does not fare so well; the physical properties of the cloth dictate that the superhero will have moved a large distance before the bottom edge of the cape “realizes” that the top edge around his neck has moved. This delay is caused because forces do not propagate instantly through cloth. Equivalently, one can think of the cape suddenly weighing 100 times more than it used to, so it initially stretches to many times its normal length.
While physically correct, this is not the desired behavior of the cape. Because of this effect, an animator must change the response of the dynamic elements to achieve the desired result. As discussed above, the dynamic elements are oftentimes complex and difficult for the animator to control. This is often frustrating for the animator, since the animator has achieved the proper motion of the kinematic elements and should not have to deal with unrealistic motion of the dynamic elements that are simulated by physically-based numerical techniques.